A "Slinky" is a toy - see here: slinky - Google Image Search
Student or Learner
"As everyone inched towards the one standing before him, the original lazy line has collapsed in on itself like a Slinky."
What does this part "the original lazy line has collapsed in on itself like a Slinky" mean?
Is there such a phrase "collapse in on itself"? Can "in" and "on" be used together this way?
As for the "lazy" line collapsing in on itself, imagine a Slinky, how it works - it all comes tumbling down or it collapses on itself once it falls from the stairs for example. As the people in your sentence move forward in the line, the line pushes itself onwards and onto its own head. Like a slinky.